Structural health monitoring is used in many different industries for analyzing stress and other material characteristics of structures. Stress field measurement techniques can be used, for example, for analyzing railroad wheels and tracks, civil structures such as dams and bridges, and other applications that require continual stress monitoring over the life of the structure. Monitoring stress in these structures is important for a variety of reasons, including the performance of accurate preventative maintenance before degradation or failure of the structure.
Destructive techniques are sometimes utilized for performing stress monitoring of structures. One such method utilizes strain gauges which are nondestructive in the sense that they must be affixed to a structure. Strain gauges measure the strain at a point on a structure that can be associated with the structure's stress field. Due to their permanent attachment at discrete localized positions, however, strain gauges are often insufficient for gathering stress information representative of the entire structure. In addition, the permanent placement of strain gauges on structures is often laborious and costly when dealing with calibration and general maintenance. In the monitoring of stresses in structures such as railroad rails, for example, the use of strain gauges is often cost prohibitive due to the excessive number of gauges necessary to acquire an accurate understanding of the variation of stresses and strains that may exist along the length of a rail.
Nondestructive evaluation methods such as X-ray and neutron diffraction techniques are capable of monitoring localized changes in strain and for measuring stress fields, but are often impractical due to their high cost and portability constraints. Nondestructive evaluation methods such as ultrasonic techniques have also been developed and are used in an effort to supplant the inadequacies of strain gauges and to provide portable systems at lower costs compared to diffraction techniques. In particular, ultrasonic evaluation techniques based on the theory of acoustoelasticity have been used to monitor stresses in loaded structures. Acoustoelasticity describes the change in velocity of elastic wave propagation in a material under an applied stress. Ultrasonic techniques based on the theory of acoustoelasticity have been employed to relate applied stresses to the change in velocity of elastic waves. These relative changes in wavespeed can also be related to strains occurring in the structure.
The practical applicability of ultrasonic evaluation methods based on wavespeed calculations is somewhat limited. To perform these calculations, a pitch-catch configuration of ultrasonic transducers comprising source and receiver transducers separated by a specified distance is commonly used. The amount of time needed for a wave to traverse this distance is directly proportional to the wavespeed and is related to the material's stress state through the acoustoelastic theory. Generally, the separation distance needs to be large enough to allow for sufficient resolution of the received signals. Furthermore, the surface geometry of the structure being analyzed needs to be uniform to prevent edge reflection effects. As a result, these factors place constraints on the shape and dimensions of the structure that can be measured. Another issue is the limited resolution observed over practical ranges of stresses. This limitation can present measurement and calibration issues when correcting for temperature gradients and elongation effects within the structure.